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Messages - willpirkle

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Schematics and Layouts / Re: Schematic sites for different brands of amps
« on: January 25, 2020, 11:07:41 AM »

Tubes and Hybrids / Re: 12 v tube preamp
« on: January 20, 2020, 01:27:15 PM »
That’s pretty cool, and only $13. Looks to be another switching charge multiplier. That site has a bunch of kits that look to range from beginner-type too so you might start there? I’m in solid-state mode these days, so only looking at that stuff right now.

Tubes and Hybrids / Re: 12 v tube preamp
« on: January 19, 2020, 05:05:45 PM »
Here’s the screamertube I built many years ago that might split the difference and give you something to experiment with that isn’t overly complicated. Note that there is a small error concerning the heaters - they should be wired in series, not parallel (causes excess heat). The circuit was changed, but not the blog.

Disclaimer: tube purists will hate it, but it was a fun project to put together!

Tubes and Hybrids / Re: 12 v tube preamp
« on: January 19, 2020, 03:32:42 PM »
PAIA used to sell just the PCB for their project kits. If you can find one of those, could be do-able! And I’d guess someone is probably cloning the PCB by now.

Tubes and Hybrids / Re: 12 v tube preamp
« on: January 19, 2020, 10:23:38 AM »
This would be considered starved plate and the B+ supply is +15V and not internally boosted, but the design does not include biasing so the first half 12ax7 only amplifies the bottom (-) half of the waveform, which is supplied via the boost amp. The second half 12ax7 reamplifies the flipped, attenuated signal (100K/4.7K resistor divider). I ran a SPICE simulation this morning and attached the results. The lowboost.png file is the output of the two tube halves (top is 1st half, bottom is the 2nd tube output) with a small input signal (low boost knob setting), hiboost.png is the same thing with a massively boosted input, and combogain.png is a combination plot. The harmonic distortion was 38% for the most distorted output.

I am not a fan of this design and I agree with Enzo - it does *something* --- but there is no duty cycle modulation you expect out of a true high-gain cascaded tube preamp when you overdrive the input, and it's going to add noise.

Have you looked at the classic PAIA Stack in a Box (SIAB)? it used a switching charge multiplier circuit to get the plate voltage up from 15V to almost 50V or so, getting it out of starved-plate region. I never built it, but it does seem to have a following of sorts.

Liquids + electronics + pedalboards + bar gigs = good idea?

Definitely fun idea.

Amplifier Discussion / Joël Huser amp module
« on: January 13, 2020, 07:42:01 PM »
This popped up on my LinkedIn feed, thought you might like it. MOSFET power amp section, claims 242W into 4 ohms. No connectors, leads solder directly to traces. I have not heard it, nor tried it, nor bought it. At the bottom of the linked page is a google drive link with the schematic and PCB layout info. Purely FYI.

Two years ago, one of my students did his senior project with a NuTube (which was practically brand new at the time, and he was able to get free parts from Korg somehow). The project was a guitar preamp, and he had nothing but problems with it; noise levels were awful, and it took a lot of shielding and HF compensation to keep the thing from squealing/hissing. Now, the student did not lay out a proper PCB (against my protests, but he ultimately ran out of time) and tried using prototype-boards (solder-type) but man, he had a ton of noise problems with that thing. Perhaps Korg has some (actual) reference designs now that address the noise issues. I got the impression that it was a sort of gimmick device.

Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 30, 2019, 11:03:25 AM »
Agree with g1 — the subbed parts are not identical with respect to the built-in base-emitter shunt resistors (which are slightly larger in the new part) and the abs max Ib, which is more than double the original (0.5A vs. 0.2A). You may need to push the bases a bit harder on the new parts. Following up on g1’s question, are your output Vbe’s healthy at idle?

Amplifier Discussion / Re: 1988 Randall RG100ES
« on: December 26, 2019, 10:11:02 PM »
The case is the collector, so yes it should be insulated if the heat sink is connected to chassis ground, which I’m assuming it is. I’d always use the insulator.

Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 26, 2019, 09:58:06 PM »
The duty cycle shift is apparent. Is it 50% up to clipping point, then as you push the signal up, the duty cycle keeps getting farther away from 50%?. Or, is it always off like this, even when not clipping? Did you solve the bias issue where the voltage-drop was 10x off?

Amplifier Discussion / Re: choosing a SS amp design to build
« on: December 24, 2019, 08:49:18 PM »
LM1875 also really well documented, many app notes and design examples. Agree with Peavey Rage suggestion; relatively simple, and has the super-distortion preamp, which makes a great little distortion box on its own. Great first amp (or dist box) project idea!

Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 21, 2019, 09:54:43 PM »
Here’s the preamp (shows letters).

Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 21, 2019, 07:02:39 PM »
I have a ‘78 210G which is nearly identical in the power amp, and attached that one (way easier to read). These are just for reference - hope they help someone. The 210G has identical output and bias transistors but emitter resistors are 0.15

Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 20, 2019, 09:10:14 PM »
Yes, picture of x-over distortion. Right at 0V or biased?

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